1994A&A...285..551V

A sample of 22 stars with infrared excess emission and many of which have supergiant-like spectra was surveyed in the submillimeter continuum at 438, 761 and 1100µm using the James Clerk Maxwell Telescope (JCMT, Mauna Kea, Hawaii). Most of the sources are at high galactic latitudes but stars with similar properties at lower galactic latitudes are also included. For 16 sources it is believed that they are post Asymptotic Giant Branch stars (post-AGB stars) and 6 of them are detected. The rest of the sample consists of 2 binaries, 1 Herbig Ae star, 1 Luminous Blue Variable and 2 sources for which the post-AGB nature is uncertain. The observations are combined with optical, near infrared and IRAS data and are fitted with a spherically symmetric and optically thin dust shell model. It is found that a large fraction of the excess emission originates from relatively hot dust near the star. Only 2 sources lack such a hot dust component. For all stars with a hot dust component it was found that the emission longward of 60µm cannot be explained with only one dust shell. Several possible explanations are discussed and it is concluded that the extra excess at far IR and submm wavelengths is caused by a second colder and thus more distant dust component. For the post-AGB stars this component is believed to be the remnant AGB shell. The relatively low stellar temperatures (<10,000 K) in combination with the relatively large ages of the remnant AGB shell (>10³ yr) confirm earlier suggestions that many of these stars are very slowly evolving towards higher temperatures, as expected for low mass post-AGB stars. Evidence was found that stars may stop losing mass and evolve off the AGB at temperatures below 5,000 K (as assumed for the Schoenberner tracks) causing a slower evolution towards higher temperatures.